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Motec M130 - Why would changing the Fuel Mixture Aim affect Fuel Closed Loop Trim

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I am using the Driver Switch to call up a "Lean of Peak" Fuel Mixture Aim table which is aimed at achieving maximum fuel economy in a modified NA Subaru EJ25 when cruising in a light plane.

I understood that the point of VE tuning was to enable fuel changes to be made without re-tuning. However the change to Fuel Mixture Aim is accompanied by a significant increase in the closed loop trims.

I am not sure if I need to tune another VE table in the "B" page called up by the Driver Switch when selecting for LoP or if there is another explanation. I attach a log file image showing the situation.

Attached Files

Have you spent time tuning the "Engine Charge Cooling Gain"? This is to account for the difference in combustion temperatures as the mixture is changed. You tune this by staying on one cell and deliberately changing the Fuel Mixture Aim, and seeing if the mixture changes by the correct amount. If not, you change the Engine Charge Cooling Gain (and re-visit the VE table for that cell and repeat). If you pay attention to how much you needed to change the VE table, I think you could apply this to the entire table to get closer.

In your case, just paying attention to the fuel trim could tell you how much to change

Another thought -- are you sure the fuel properties are correct? I see that the properties for the "Gasoline 98 octane 1.0" (select this and then choose manual for Fuel Properties Calibration), has a density of 755 kg/m^3 (.755 specific gravity). A bit of googling and I found this comment with various specific gravity values for 100LL Avgas.

It seems like this would be a linear change, so if you're counting on this feature and don't want to use closed loop to correct it, then I would try setting the correct fuel properties for the fuel you intend to use.

Thanks David

I had a go at setting Engine Charge Cooling Gain some time ago and am using 98 Octane RON fuel.

I have since added a second lambda sensor on the even cylinder bank but need to add it to the logging. Maybe it has something to do with closed loop trims on No 1 cylinder bank increasing. I believe that the trims climb from around 7% to 10% as the MAP approaches 80.


Actually you are spot on.

The Engine Charge Cooling Gain was set for the previous motor which was a stock EJ253 running AVLS heads. The current motor is a custom build with a longer stroke and longer duration fixed camshaft heads.

There is a good chance that the tuner who helped with the instal did not revisit the gain. I will do it again now and if it is wrong will retune.

Adjusted the Engine Charge Cooling Gain Compensation to .9 to achieve matching values at Lambda 1.0 and 0.9 but the closed loop trim issue remains.

There is also a more worrying event occurring where the exhaust lambda followed by the closed loop aim enters extreme fluctuations up to the 25% trim limit currently set. The 1.10 Fuel Mixture Aim table called up by the driver switch extends to 5000 rpm and the reading when fluctuations begin was around 4650 so I don't see how the fluctuations could be triggered by the edge of the table.

Image attached - the driver switch was activated at 18.20.

Attached Files

When I see wildly varying lambda, this usually indicates a misfire, or ignition cut situation. I notice the exhaust temps were climbing in the lean condition, do you have any engine speed limits that would activate on high EGTs?

I would suggest you right click on the graph and choose Display->Show Status/Errors (short cut, press the "E" key). This will allow you to identify where the status changes, with the values list shown (View->Show Values, or "V" key), scroll down to the status section, to identify what changes are happening.

If you wish you could put a logfile on Google Drive or Dropbox, and send me ( a link and I would take a look. Pictures are nice, but I could do a lot more with the actual logfile to examine.

I think that you have nailed it again. Quite some time ago I set an exhaust temperature fuel compensation for around 845 C (1550 F ?) and have not thought any more about it because the aviation EGT gauge (EI) with probe per cylinder rarely exceeds 1400 F. The higher reading from the Motec sensor just before the fluctuation would be about where the exhaust temperature fuel compensation table calls for Lambda .78 (previous cells are 2.00).

Thanks so much for this.

I have endeavoured to send a link to the log file at Dropbox.